Thermoelectric cooling of low-noise amplifier transistors in wireless communications networks

ABSTRACT

A base station for a wireless communications system having a tower-mounted amplifier system with a low-noise amplifier transistor and a thermoelectric cooler that reduces the operating temperature of the low-noise amplifier transistor. The amplifier system has additional heat-generating components, such as filters, and additional electrical components mounted on a substrate to which the low-noise amplifier transistor is mounted. These heat-generating components are thermally isolated from the cold side of the thermoelectric cooler. As a result, the cooling capacity and electrical power requirement for the thermoelectric cooler is significantly reduced because only the low-noise amplifier transistor is cooled.

FIELD OF THE INVENTION

This invention relates generally to wireless communications networksand, in particular, to reducing the operating temperature of low-noiseamplifier transistors used in wireless communications networks.

BACKGROUND OF THE INVENTION

Wireless communication networks divide a coverage area into multiplecells each arranged to communicate with mobile stations (e.g., mobilephones) with minimal interference between the cells. A mobile stationcrossing the coverage area has its communications handed-off betweenadjacent cells. Each of the multiple cells is typically served by a basestation having a transceiver that communicates with the mobile stationvia an antenna situated on a tower.

Tower-mounted amplifiers (TMA's) used with the base station antennaimprove the sensitivity of base station with respect to amplifiersystems located at the base of the tower. Signals received by theantenna are amplified by the TMA before being transmitted over atransmission line to a radio of the base station. As a result, thesignal-to-noise ratio of the signal communicated to the radio ispreserved as the line losses reduce the level of the amplified signaltransmitted to the radio. Preserving the signal-to-noise ratio alsoreduces the number of base stations required to cover a given coveragearea by extending their range.

To optimize sensitivity and improve performance of the TMA, it isdesirable that its low-noise amplifier transistor have a minimized noisefigure. A conventional approach for enhancing the performance ofheat-sensitive electronic equipment, such as low-noise amplifiertransistors, is to cool the components of the equipment using athermoelectric cooler. Thermoelectric coolers constitute solid stateheat pumps that may be used to extract heat from electronic equipment.Often, a thermoelectric cooler includes a cold side that is placed inheat transfer communication with the chassis or housing of a completeelectronic device, a hot side from which transferred heat is dissipated,and a thermoelectric module that transfers heat from the cold side tothe hot side.

This conventional approach for reducing the operating temperature ofelectronic devices suffers from certain deficiencies. In manyconventional arrangements, the thermoelectric cooler must cool anenclosure housing the electronic device and various heat-generatingcomponents inside the enclosure. For example, a TMA incorporatesheat-generating components, such as filters, in addition to thelow-noise amplifier transistor of the TMA. As a result, the coolingcapacity of the thermoelectric cooler must accommodate heat generated bycomponents of the TMA in addition to the heat generated by the low-noiseamplifier transistor. Increasing the required cooling capacity of thethermoelectric cooler increases its cost and also increases theelectrical power required to operate the thermoelectric cooler.Moreover, a ventilation fan may be required to convectively cool a heatsink thermally coupled with the hot side of the thermoelectric cooler.

Therefore, it would be desirable, among other things, to address coolingissues associates with TMAs and to reduce the cooling capacity of athermoelectric cooler used to reduce the operating temperature oflow-noise amplifier transistors used in wireless communicationsnetworks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view depicting the connection between a basestation and a tower-mounted amplifier system in accordance with theprinciples of the invention;

FIG. 2 is a diagrammatic view of a portion of the tower-mountedamplifier system of FIG. 1;

FIG. 3 is a side view of the low-noise amplifier transistor and thethermoelectric cooler of FIG. 2;

FIG. 4 is a side view similar to FIG. 3 of an alternative embodiment inaccordance with the principles of the invention; and

FIG. 5 is a side view similar to FIG. 3 of another alternativeembodiment in accordance with the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a base station, generally indicated byreference numeral 10, of a wireless communications system often includesa transceiver radio 12 housed inside a ground-level shelter 13, anantenna 14, and a tower-mounted amplifier module 16 enclosed inside of atower housing 18. The antenna 14 and tower-mounted amplifier module 16are each mounted to a tower 20 or other support structure, such as abuilding. A transmission link 22, such as a transmission line orwireless link, couples the tower-mounted amplifier 16 to the radio 12for electrically coupling tower-mounted amplifier module 16 with radio12. Also provided inside the tower housing 18 are diplexing filters 24,26, 29 effective for reducing out of band interference and to enablesharing of the antenna 14 and transmission link 22 for both transmittedand received signals transferred between the radio 12 and the antenna14. Collectively, the filters 24, 26, 29 may function as a dualdiplexer. Tower housing 18 is typically formed of a material having ahigh thermal conductivity, such as a metal.

The antenna 14 contains radiating elements of any type suitable for usein a wireless communication network. Suitable radiating elements forantenna 14 include, but are not limited to, monopole elements, dipoleelements, loops, slots, spirals or helices, horns, and microstrippatches.

With reference to FIGS. 1 and 2, the tower-mounted amplifier module 16includes a low-noise amplifier transistor 28 disposed on a substrate 30,such as a printed circuit board. The low-noise amplifier transistor 28may have a single, duplex or dual duplex configuration as understood bypersons of ordinary skill in the art. One exemplary low-noise amplifierPHEMT transistor suitable for use as low-noise amplifier transistor 28consistent with the invention is the Model ATF-54143 FET transistoravailable from Agilent Technologies (Palo Alto, Calif.), which has a 0.5dB noise figure and a 16.6 dB associated gain. The side of the substrate30 to which the low-noise amplifier transistor 28 is mounted isgenerally constructed from a material having a relatively low thermalconductivity and is not an efficient in-plane conductor of heat. Forexample, a printed circuit board is typically a layered compositeconsisting of a thin copper foil ground plane and a significantlythicker, glass-reinforced polymer, such as the epoxy-based organic FR-4having a thermal conductivity of about 0.25 W/mK.

Substrate 30 includes a plurality of, for example, two electricalcomponents 31 and various electrically conductive paths (not shown), asunderstood by a person of ordinary skill in the art, required foroperation of the amplifier module 16. During operation, the activeelements of the low-noise amplifier 28 and the electrical components 31generate heat and present individual heat loads. The low-noise amplifiertransistor 28 and the electrical components 31 are often encapsulatedinside a surface-mount package formed from a thermoplastic resin.

In accordance with the principles of the invention and with reference toFIGS. 2 and 3, the tower-mounted amplifier module 16 includes athermoelectric cooler 32 that extracts or withdraws heat from thelow-noise amplifier transistor 28. The thermoelectric cooler 32 includesa lower support plate 34 and an upper support plate 36 between whichmultiple thermoelectric elements 38 are disposed. The thermoelectricelements 38 consist of an array of dissimilar n-type and p-typesemiconductors thermally joined in parallel and electrically joined inseries at both ends to form a couple. The thermoelectric cooler 32,which operates by the Peltier effect, converts electrical energy to heatpumping energy.

Direct-current power applied across the lower and upper support plates34, 36 induces heat flow from the upper support plate 36 through thethermoelectric elements 38 to the lower support plate 34 as thethermoelectric elements 38 convert electrical energy to heat pumpingenergy. As heat is continuously transferred, the upper support plate 36defines a cold side and the lower support plate 34 defines a hot side.The upper support plate 36 is in thermal contact or heat transfercommunication with the low-noise amplifier transistor 28. To that end, athermally conductive element 39 is provided in the substrate 30 betweenthe thermoelectric cooler 32 and the low-noise amplifier transistor 28.In one embodiment, the thermally conductive element 39 may be aplurality of thermal vias extending through substrate 30 and formed of amaterial having a high thermal conductivity, such as copper having aroom temperature thermal conductivity of about 400 W/mK. Alternatively,the thermally conductive element 39 may be provided as an insertpositioned in a cut-out in the substrate 30 and formed from a ceramic,such as alumina having a room temperature thermal conductivity of about18 W/mK, aluminum nitride (about 200 W/mK), or beryllium oxide (about300 W/mK), or a metal, such as copper (about 400 W/mK).

The upper support plate 36 absorbs heat generated by the active devicesof the low-noise amplifier transistor 28. Because the upper supportplate 36 is in thermal contact only with the low-noise amplifiertransistor 28, the cooling capacity of the thermoelectric cooler 32 isreduced when compared with conventional tower-mounted amplifiers inwhich a thermoelectric cooler cools the tower housing, the filters, andother heat generating elements inside the tower housing in addition tothe low-noise amplifier transistor 28. The transferred heat isdissipated from the lower support plate 34 to the surroundingenvironment. The lower support plate 34 includes a heat sink 42 toincrease the heat dissipation efficiency by maintaining the temperatureof the lower support plate 34 as low as possible. The heat sink 42 maybe any conventional structure, such as a heat spreader, a cold plate, ora convective fin stack, or the tower housing 18 of the electronicdevice.

With continued reference to FIGS. 2 and 3, the substrate 30 has a lowthermal conductivity so that little heat is transferred throughsubstrate 30 from the electrical components 31 or the tower housing 18to the thermoelectric cooler 32. In an alternative embodiment, thesubstrate 30 may incorporate a thermal insulator 40 that assists inthermally insulating the thermoelectric cooler 32 against heat generatedby the electrical components 31. Specifically, the thermal insulator 40represents a discontinuity in a heat flow path from electricalcomponents 31 to the upper support plate 36 of the thermoelectric cooler32. The thermal insulator 40 is any material having a lesser thermalconductivity than substrate 30. In one embodiment of the invention, theheat insulator 40 is an air gap, which disrupts conductive pathways tothe upper support plate 36, as air is an effective thermal insulator.

The filters 24, 26, 29 are separated from the substrate 30 by respectiveair gaps 25, 27, 33, each of which operates as an effective thermalinsulator eliminating pathways for heat conduction to the upper supportplate 36. As a result, heat generated by the filters 24, 26, 29 cannotflow to the low-noise amplifier transistor 28 and, therefore, is notextracted by thermoelectric cooler 32. Air gaps 25, 27 33 are eachunderstood to be a separation between any two points of thecorresponding one of filters 24, 26, 29 and the substrate 28, which arethree-dimensional objects. Moreover, the upper support plate 36 of thethermoelectric cooler 32 is thermally isolated from the tower housing 18by substrate 30 and also by an air gap 35 separating tower housing 18from substrate 30 and upper support plate 36 so that heat is nottransferred from the tower housing 18 to the upper support plate 36. Airgap 35 is understood to extend about all confronting surfaces of thetower housing 18 and substrate 30, which are three-dimensional objects.In certain embodiments of the invention, the air gap 35 may be absent sothat the thermal insulation is provided exclusively by the substrate 30.It follows that, due to the thermal insulation against heat transfer,the thermoelectric cooler 32 does not have to dissipate heat or, at theleast, a significant heat load, transferred from filters 24, 26, 29 ortower housing 18.

With continued reference to FIGS. 2 and 3, a tower-mountedcontroller/power supply 44 supplies operating power to thethermoelectric cooler 32 over cables 46, 48 and may also be positionedinside of the tower housing 18. The thermoelectric cooler 32 andcontroller/power supply 44 may be packaged in a single chip device. Atemperature sensor 50, such as a thermistor or platinum resistancetemperature detector, monitors the temperature of low-noise amplifiertransistor 28 and/or thermoelectric cooler 32 and provides informationcharacterizing the temperature via a cable 52 to the controller/powersupply 44. The heat transfer rate of the thermoelectric cooler 32 isadjusted by controlling the power supplied by controller/power supply 44to the upper and lower support plates 34, 36. In this way, feedbackcontrol can be accomplished so that the temperature of the low-noiseamplifier transistor 28 can be adjusted to, and maintained at, a desiredoperating temperature.

An exemplary thermoelectric cooler suitable for use in the invention isthe Model SP5060 thermoelectric cooler commercially available fromMarlow Industries Inc. (Dallas, Tex.), which is capable of providing amaximum temperature drop for an unloaded state of 68.5° C.-dry N₂ fromthe cold side to the hot side with the hot side at a temperature of 27°C. and a maximum temperature drop for an unloaded state of 88.5° C.-dryN₂ from the cold side to the hot side with the hot side at a temperatureof 85° C.

A cover 53 of a material having a low thermal conductivity may bepositioned on the substrate 30 so as to enclose the low-noise amplifiertransistor 28. The cover 53 reduces heat transferred from the ambientatmosphere inside the tower housing 18 to the low-noise amplifiertransistor 28 and, therefore, to the upper support plate 36 of thethermoelectric cooler 32. The thermal insulation provided by the cover53 significantly reduces the heat load that must be dissipated by thethermoelectric cooler 32, as the ambient atmosphere inside the towerhousing 18 may be heated when deployed on tower 20 and during operation.

In use, the thermoelectric cooler 32 is energized by power supplied bythe tower-mounted controller/power supply 44, which causes heat transferor heat flow from the upper support plate 36 through the thermoelectricelements 38 to the lower support plate 34 and reduces the temperature ofthe upper support plate 36. A temperature gradient exists between thelower and upper support plates 34, 36 that increases in a direction fromthe upper support plate 36 to the lower support plate 34. The low-noiseamplifier transistor 28 of the tower-mounted amplifier module 16presents a heat load during operation. Heat is transferred from thelow-noise amplifier transistor 28 through the thermally conductiveelement 39 to the upper support plate 36, which reduces the operatingtemperature of the low-noise amplifier transistor 28. The temperaturesensor 50 provides temperature information to the controller/powersupply 44, which in turn regulates the operating power delivered to thethermoelectric cooler 32 for maintaining the low-noise amplifiertransistor 28 at a constant operating temperature that is significantlyless than the ambient temperature of the surrounding environment.

The tower housing 18, filters 24, 26, 29 and electrical components 31are not cooled by the thermoelectric cooler 32. In particular, coolingthe filters 24, 26, 29 and the electrical components 31 is not necessarybecause the heat generated by the filters 24, 26, 29 and electricalcomponents 31 does not produce as much temperature-dependent noise and,therefore, do not significantly contribute to degrading thesignal-to-noise ratio of the signal transmitted from the antenna 14 tothe radio 12.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 3 and in accordance with an alternative embodiment ofthe invention, an opening 54 is provided in the substrate 30. Opening 54is dimensioned such that upper support plate 36 may be placed intodirect contact with the low-noise amplifier transistor 28 without anyintervening thermally conductive element 39 (FIG. 3).

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 3 and in accordance with an alternative embodiment ofthe invention, upper support plate 36′ is patterned with metallizationtraces 56, 58 and 60 electrically coupling the low-noise amplifiertransistor 28 and the electrical components 31 with one another and theantenna 14, transmission link 22, and the filters 24, 26, 29 (FIG. 1).Thereby, the substrate 30 (FIG. 2) may be reduced in size or eliminatedin its entirety. The metallization traces 56, 58 and 60 are surroundedby a non-conductive portion 62 of the upper support plate 36′.

With renewed reference to FIG. 1, the transceiver radio 12 is mountedinside ground-level shelter 13, which operates as a support structure,and houses a substrate 64, similar to substrate 30. A power amplifiertransistor 66 mounted to the substrate 64 generates heat when energizedand operating. In accordance with the principles of the invention, theoperating temperature of the power amplifier transistor 66 may bereduced by a operation of a thermoelectric cooler 68, as describedherein with regard to low-noise amplifier transistor 28 andthermoelectric cooler 32. Heat generated from an adjacent electroniccomponent 70 of the transceiver radio 12 is not transferred to thethermoelectric cooler 68. For example, electronic component 70 may beseparated from the power amplifier transistor 66 and the thermoelectriccooler 68 by an air gap 72 that limits heat transfer. Alternatively, thesubstrate 64 may be formed from a material characterized by a relativelylow thermal conductivity. Therefore, the thermoelectric cooler 68 isthermally insulated from electronic component 68 but effectively coolsthe power amplifier transistor 66. The thermoelectric cooler 68 has anupper support plate or cold side (not shown but similar to upper supportplate 36 of thermoelectric cooler 32) that may be coupled in heattransfer communication with power amplifier transistor 66 by a thermallyconductive element (not shown but similar to thermally conductiveelement 39) or directly coupled in heat transfer communication with thepower amplifier transistor 66, as described herein with regard tolow-noise amplifier transistor 28. The transceiver radio 12 constitutesa housing that is thermally insulated from the cold side by a heatinsulator, such as an air gap 74 and/or the substrate 64 of lowerthermal conductivity.

In accordance with the principles of the invention, cooling thelow-noise amplifier transistor of the tower-mounted amplifier modulereduces the noise figure and concomitantly increases the sensitivity ofthe low-noise amplifier transistor. The thermoelectric cooler cools theactive devices of the low-noise amplifier transistor without thenecessity of any moving components so as to increase reliability of thecooling process and to decrease the cost of operation. In one aspect, aventilation fan not is required, as is conventional, to provide a flowof air for convective cooling of the hot side of the thermoelectriccooler.

In addition, the cooling capacity and electrical power requirement forthe thermoelectric cooler is significantly reduced because only thelow-noise amplifier transistor is cooled so as to decrease the thermalload that must be dissipated by the thermoelectric cooler. Nonetheless,cooling only the low-noise amplifier transistor active devices isadequate for effectively reducing the noise figure of the tower-mountedamplifier module as electrical components on the substrate carrying thelow-noise amplifier transistor and filters and the like of thetower-mounted amplifier system do not significantly degrade the systemsignal-to-noise ratio. In one particular embodiment, the thermoelectriccooler is effective to reduce the noise figure of the low-noiseamplifier transistor by about 0.5 dB, which reduces the number of basestations required to service a coverage area.

Alternatively, cooling only the low-noise amplifier transistor to reducethe noise figure permits the selection of a low-noise amplifiertransistor having a larger room temperature noise figure and a smallerfilter having a greater loss. This would effectively reduce the cost ofthe low-noise amplifier transistor as the specifications therefore wouldbe less stringent.

While the present invention has been illustrated by a description ofvarious preferred embodiments and while these embodiments have beendescribed in considerable detail in order to describe the best mode ofpracticing the invention, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications within the spirit andscope of the invention will readily appear to those skilled in the art.The invention itself should only be defined by the appended claims,

1. An amplifier module for attachment to a support structure of a basestation in a wireless communications network, the module comprising: asubstrate; a heat-generating electrical component coupled to saidsubstrate; a low-noise amplifier transistor coupled to said substrateand capable of generating heat; and a thermoelectric cooler positionedproximate said substrate, said thermoelectric cooler having a cold sidethermally coupled with said low-noise amplifier transistor, said coldside being cooled for extracting heat from said low-noise amplifiertransistor, and said cold side being thermally isolated from saidelectrical component.
 2. The amplifier module of claim 1 furthercomprising: a heat-generating filter thermally isolated from said coldside by a heat insulator.
 3. The amplifier module of claim 2 whereinsaid heat insulator is an air gap.
 4. The amplifier module of claim 1wherein said substrate further comprises: a heat insulator separatingsaid electrical component from said cold side.
 5. The amplifier moduleof claim 4 wherein said substrate is formed from a material having a lowthermal conductivity, and said heat insulator is a portion of saidsubstrate separating said electrical component from said cold side. 6.The base station of claim 4 wherein said heat insulator is an air gapprovided in said substrate between said electrical component and saidcold side.
 7. The amplifier module of claim 1 wherein said substratefurther comprises: a thermally conductive element coupling said coldside in heat transfer communication with said low-noise amplifiertransistor.
 8. The amplifier module of claim 1 wherein said cold side isdirectly coupled in heat transfer communication with said low-noiseamplifier transistor.
 9. The amplifier module of claim 1 furthercomprising a housing enclosing said substrate, said housing beingthermally isolated from said cold side.
 10. The amplifier module ofclaim 9 further comprising: a heat insulator separating said housingfrom said cold side.
 11. The amplifier module of claim 10 wherein saidheat insulator is an air gap between said housing and said cold side.12. An amplifier module for attachment to a support structure for a basestation in a wireless communications network, the module comprising: ahousing capable of being mounted to said support structure; said; alow-noise amplifier transistor positioned inside said housing andcapable of generating heat; and a thermoelectric cooler positionedinside said housing, said thermoelectric cooler having a cold sidethermally coupled with said low-noise amplifier transistor, said coldside being cooled for extracting heat from said low-noise amplifiertransistor, and said cold side being thermally isolated from saidhousing.
 13. The amplifier module of claim 12 wherein further comprisinga heat insulator separating said housing from said cold side.
 14. Theamplifier module of claim 13 wherein said heat insulator is an air gapbetween said housing and said cold side.
 15. A transceiver radio for abase station of a wireless communications network, comprising: asubstrate; a power amplifier transistor coupled to said substrate andcapable of generating heat; a heat-generating electrical componentcoupled to said substrate; and a thermoelectric cooler positionedproximate said substrate, said thermoelectric cooler having a cold sidethermally coupled with said power amplifier transistor, said cold sidebeing cooled for extracting heat from said power amplifier transistor,and said cold side being thermally isolated from said electricalcomponent.
 16. The transceiver radio of claim 15 wherein said substratefurther comprises: a heat insulator separating said electrical componentfrom said cold side.
 17. The transceiver radio of claim 16 wherein saidsubstrate is formed from a material having a low thermal conductivity,and said heat insulator is a portion of said substrate separating saidelectrical component from said cold side.
 18. The transceiver radio ofclaim 16 wherein said heat insulator is an air gap provided in saidsubstrate between said electrical component and said cold side.
 19. Thetransceiver radio of claim 15 wherein said substrate further comprises:a thermally conductive element coupling said cold side in heat transfercommunication with said power amplifier transistor.
 20. The transceiverradio of claim 15 wherein said cold side is directly coupled in heattransfer communication with said power amplifier transistor.